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Summary
IGNEOUS ROCKS AND METAMORPHIC ROCKS DERIVED FROM
IGNEOUS “parents” make up about 95 percent of Earth’s
crust. Furthermore, the mantle, which accounts for more
than 82 percent of Earth’s volume, is composed entirely of
igneous rock. Thus, Earth can be described as a huge
mass of igneous rock covered with a thin layer of
sedimentary rock and having a relatively small iron-rich
core. Consequently, a basic knowledge of igneous rocks is
essential to our understanding of the structure,
composition, and internal workings of our planet.
Igneous Rocks and Intrusive Activity
• Igneous rock forms from magma that cools and solidifies in a process
called crystallization.
• Sedimentary rock forms from the lithification of sediment.
• Metamorphic rock forms from rock that has been subjected to great
pressure and heat in a process called metamorphism.
The rate of cooling of magma greatly influences the size of mineral crystals in
igneous rock. There are four basic igneous rock textures:
(1) Fine-grained,
(2) Coarse-grained,
(3) Porphyritic (rock texture, typically found in volcanic rocks, containing distinct crystals or
crystalline particles embedded in a fine-grained groundmass
(4) Glassy Texture.
Igneous rocks are classified by their texture and mineral composition.
Igneous rocks are divided into broad compositional groups based on the percentage
of dark and light silicate minerals they contain.
• Felsic rocks (e.g., granite and rhyolite) are composed mostly of the
light-colored silicate minerals potassium feldspar and quartz.
• Rocks of intermediate composition (e.g., andesite) contain plagioclase
feldspar and amphibole.
• Mafic rocks(a group of dark-colored, mainly ferromagnesian minerals such
as pyroxene and olivine. (e.g., basalt) contain abundant olivine,
pyroxene(large class of rock-forming silicate minerals, generally containing calcium, magnesium, and iron and
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typically occurring as prismatic crystals)
, and calcium feldspar
The mineral makeup of an igneous rock is ultimately determined by the
chemical composition of the magma from which it crystallized. N. L.
Bowen showed that as magma cools, minerals crystallize in an orderly
fashion. Magmatic differentiation changes the composition of magma and
causes more than one rock type to form from a common parent magma.
Igneous rocks (ignis = fire) form as molten rock cools and solidifies. Considerable
evidence supports the idea that the parent material for igneous rocks, called
magma, is formed by melting that occurs at various levels within Earth’s crust and
upper mantle to depths of perhaps 250 kilometers (about 150 miles).
Once formed, a magma body buoyantly rises toward the surface because it is less
dense than the surrounding rocks. (When rock melts it takes up more space and,
hence, it becomes less dense than the surrounding solid rock.) Occasionally molten
rock reaches Earth’s surface where it is called lava .
Sometimes lava is emitted as fountains that are produced when escaping gases
propel it from a magma chamber. On other occasions, magma is explosively
ejected, producing dramatic steam and ash eruptions. However, not all eruptions
are violent; many volcanoes emit quiet outpourings of very fluid lava.
The Nature of Magma:
Magma is completely or partly molten rock, which on cooling solidifies to form an
igneous rock composed of silicate minerals. Most magmas consist of three distinct
parts—a liquid component, a solid component, and a gaseous phase.
The liquid portion, called melt, is composed mainly of mobile ions of the eight
most common elements found in Earth’s crust—silicon and oxygen, along with
lesser amounts of aluminum, potassium, calcium, sodium, iron, and magnesium.
The solid components (if any) in magma are silicate minerals that have already
crystallized from the melt. As a magma body cools, the size and number of crystals
increase. During the last stage of cooling, a magma body is like a “crystalline
mush” with only small amounts of melt.
The gaseous components of magma, called volatiles, are materials that will
vaporize (form a gas) at surface pressures. The most common volatiles found in
magma are water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2),
which are confined by the immense pressure exerted by the overlying rocks. These
gases tend to separate from the melt as it moves toward the surface (low-pressure
environment).
As the gases build up, they may eventually propel magma from the vent. When
deeply buried magma bodies crystallize, the remaining volatiles collect as hot,
water-rich fluids that migrate through the surrounding rocks. These hot fluids play
an important role in metamorphism and will be considered in Chapter 7.
In the process called crystallization, cooling reverses the events of melting. As the
temperature of the liquid drops, ions pack more closely together as their rate of
movement slows. When cooled sufficiently, the forces of the chemical bonds will
again confine the ions to an orderly crystalline arrangement. When magma cools, it
is generally the silicon and oxygen atoms that link together first to form silicon–
oxygen tetrahedra, the basic building blocks of the silicate minerals.
As magma continues to lose heat to its surroundings, the tetrahedra join with each
other and with other ions to form crystal nuclei. Slowly each nucleus grows as ions
lose their mobility and join the crystalline network.
The earliest formed minerals have space to grow and tend to have better developed
Crystal faces than do the later ones that occupy the remaining spaces. Eventually all
of the melt is transformed into a solid mass of interlocking silicate Minerals that we
call an igneous rock. As you will see, the crystallization of magma is much more
complex than just described. Whereas a simple compound, such as water, solidifies
at a specific temperature, crystallization of magma with its diverse chemistry spans
a temperature range of 200 °C, or more. In addition, magmas differ from one another
in terms of their chemical composition, the amount of volatiles they contain, and the
rate at which they cool. Because all of these factors influence the crystallization
process, the appearance and mineral make-up of igneous rocks varies widely.
Igneous Processes
Igneous rocks form in two basic settings. Magma may crystallize at depth or lava
may solidify at Earth’s surface. When magma loses its mobility before reaching the
surface it eventually crystallizes to form intrusive igneous rocks. They are also
known as plutonic rocks—after Pluto, the god of the lower world in classical
mythology. Intrusive igneous rocks are coarse-grained and consist of visible mineral
crystals. These rocks are observed at the surface in locations where uplifting and
erosion have stripped away the overlying rocks.
Igneous rocks that form when molten rock solidifies at the surface are classified as
extrusive igneous rocks. They are also called volcanic rocks—after the Roman fire
god, Vulcan. Extrusive igneous rocks form when lava solidifies, in which case they
tend to be fine-grained, or when volcanic debris falls to Earth’s surface. Extrusive
igneous rocks are abundant in western portions of the Americas where they make up
the volcanic peaks of the Cascade Range and the Andes Mountains. In addition,
many oceanic islands, including the Hawaiian chain and Alaska’s Aleutian Islands,
are composed almost entirely of extrusive igneous rocks.
From Magma to Crystalline Rock:
To better understand how magma crystallizes, let us consider how a simple
crystalline solid melts. Recall that in any crystalline solid, the ions are arranged in a
closely packed regular pattern. However, they are not without some motion—they
exhibit a sort of restricted vibration about fixed points. As temperature rises, ions
vibrate more rapidly and consequently collide with ever-increasing vigor with their
neighbors. Thus, heating causes the ions to occupy more space, which in turn causes
the solid to expand. When the ions are vibrating rapidly enough to overcome the
force of their chemical bonds, melting occurs. At this stage the ions are able to slide
past one another, and the orderly crystalline structure disintegrates. Thus, melting
converts a solid consisting of tight, uniformly packed ions into a liquid composed of
unordered ions moving randomly about. In the process called crystallization, cooling
reverses the events of melting. As the temperature of the liquid drops, ions pack
more closely together as their rate of movement slows. When cooled sufficiently,
the forces of the chemical bonds will again confine the ions to an orderly crystalline
arrangement. When magma cools, it is generally the silicon and oxygen atoms that
link together first to form silicon–oxygen tetrahedra, the basic building blocks of the
silicate minerals. As magma continues to lose heat to its surroundings, the tetrahedra
join with each other and with other ions to form embryonic crystal nuclei. Slowly
each nucleus grows as ions lose their mobility and join the crystalline network.
coarse-grained and consist of visible mineral crystals. These rocks are observed at
the surface in locations where uplifting and erosion have stripped away the overlying
rocks. Exposures of intrusive igneous rocks occur in many places, including Mount
Washington, New Hampshire; Stone Mountain, Georgia; the Black Hills of South
Dakota; and Yosemite National Park, California (FIGURE 3.2).
Igneous rocks that form when molten rock solidifies at the surface are classified as
extrusive igneous rocks. They are also called volcanic rocks—after the Roman fire
god, Vulcan. Extrusive igneous rocks form when lava solidifies, in which case they
tend to be fine-grained, or when volcanic debris falls to Earth’s surface. Extrusive
igneous rocks are abundant in western portions of the Americas where they make up
the volcanic peaks of the Cascade Range and the Andes Mountains. In addition,
many oceanic islands, including the Hawaiian chain and Alaska’s Aleutian Islands,
are composed almost entirely of extrusive igneous rocks.
Igneous Compositions:
Igneous rocks are composed mainly of silicate minerals. Chemical analyses show
that silicon and oxygen are by far the most abundant constituents of igneous rocks.
These two elements, plus ions of aluminum (Al), calcium (Ca), sodium (Na),
potassium (K), magnesium (Mg), and iron (Fe), make up roughly 98 percent, by
weight, of most magmas. In addition, magma contains small mounts of many other
elements, including titanium and manganese, and trace amounts of much rarer
elements such as gold, silver, and uranium.
As magma cools and solidifies, these elements combine to form two major groups
of silicate minerals. The dark (ferromagnesian) silicates are rich in iron and/or
magnesium and comparatively low in silica. Olivine, pyroxene, amphibole, and
biotite mica are the common dark silicate minerals of Earth’s crust. By contrast,
the light (nonferromagnesian) silicates contain greater amounts of potassium,
sodium, and calcium rather than iron and magnesium.
As a group, nonferromagnesian minerals are richer in silica than the dark silicates.
The light silicates include quartz, muscovite mica, and the most abundant mineral
group, the feldspars. Feldspars make up at least 40 percent of most igneous rocks.
Thus, in addition to feldspar, igneous rocks contain some combination of the other
light and/or dark silicates.
Granitic (Felsic) Versus Basaltic (Mafic) Compositions:
Despite their great compositional diversity, igneous rocks (and the magmas
from which they form) can be divided into broad groups according to their
proportions of light and dark minerals (FIGURE 3.3). Near one end of the continuum
are rocks composed almost entirely of light-colored silicates—quartz and feldspar.
Igneous rocks in which these are the dominant minerals have a granitic composition.
Geologists also refer to granitic rocks as being felsic, a term derived from feldspar
and silica(quartz). In addition to quartz and feldspar, most granitic rocks contain
about 10 percent dark silicate minerals, usually biotite mica and amphibole. Granitic
rocks are rich in silica (about 70 percent) and are major constituents of the
continental crust . Rocks that contain substantial dark silicate minerals and calciumrich plagioclase feldspar (but no quartz) are said to have a basaltic composition (see
Figure 3.3). Basaltic rocks contain a high percentage of ferromagnesian minerals, so
geologists also refer to them as mafic (from magnesium and ferrum, the Latin name
for iron). Because of their iron content, mafic rocks are typically darker and denser
than granitic rocks. Basaltic rocks make up the ocean floor as well as many of the
volcanic islands located within the ocean basins. Basalt also forms extensive lava
flows on the continents.
Other Compositional Groups:
As you can see in Figure 3.3, rocks with a composition between granitic and basaltic
rocks are said to have an intermediate composition, or andesitic composition after
the common volcanic rock andesite. Intermediate rocks contain at least 25 percent
dark silicate minerals, mainly amphibole, pyroxene, and biotite mica with the other
dominant mineral being plagioclase feldspar. This important category of igneous
rocks is associated with volcanic activity that is typically confined to the margins of
the continents.
Another important igneous rock, peridotite, contains mostly olivine and pyroxene
and thus falls on the opposite side of the compositional spectrum from granitic rocks
(see Figure 3.3). Because peridotite is composed almost entirely of ferromagnesian
minerals, its chemical composition is referred to as ultramafic. Although ultramafic
rocks are rare at Earth’s surface, peridotite is the main constituent of the upper
mantle.
Igneous Textures:
The term texture is used to describe the overall appearance of a rock based on
the size, shape, and arrangement of its mineral grains (FIGURE 3.4). Texture is an
important property because it reveals a great deal about the environment in which
the rock formed. This fact allows geologists to make inferences about a rock’s origin
based on careful observations of grain size and other characteristics of the rock.
Other Compositional Groups:
As you can see in Figure 3.3, rocks with a composition between granitic and
basaltic rocks are said to have an intermediate composition, or andesitic composition
after the common volcanic rock andesite. Intermediate rocks contain at least 25
percent dark silicate minerals, mainly amphibole, pyroxene, and biotite mica with
the other dominant mineral being plagioclase feldspar. This important category of
igneous rocks is associated with volcanic activity that is typically confined to the
margins of the continents. Another important igneous rock, peridotite, contains
mostly olivine and pyroxene and thus falls on the opposite side of the compositional
spectrum from granitic rocks (see Figure 3.3). Because peridotite is composed
almost entirely of ferromagnesian minerals, its chemical composition is referred to
as ultramafic.
Although ultramafic rocks are rare at Earth’s surface, peridotite is the main
constituent of the upper mantle (Figure 3.3). The percentage of silica in igneous
rocks actually varies in a systematic manner that parallels the abundance of other
elements. For example, rocks that are relatively low in silica contain large amounts
of iron, magnesium, and calcium.
By contrast, rocks high in silica contain very little iron, magnesium, or calcium but
are enriched with sodium and potassium. Consequently, the chemical makeup of an
igneous rock can be inferred directly from its silica content.
Further, the amount of silica present in magma strongly influences its behavior.
Granitic magma, which has a high silica content, is quite viscous (“thick”) and may
erupt at temperatures as low as 700 °C. On the other hand, basaltic magmas are
low in silica and are generally more fluid.
Basaltic magmas also erupt at higher temperatures than granitic magmas—usually
at temperatures between 1100 and 1250 °C and are completely solid when cooled
to 1000 °C. In summary, igneous rocks can be divided into broad groups according
to the proportions of light and dark minerals they contain. Granitic (felsic) rocks,
which are composed almost entirely of the light colored minerals quartz and
feldspar, are at one end of the compositional spectrum (see Figure 3.3). Basaltic
(mafic) rocks, which contain abundant dark silicate minerals in addition to
plagioclase feldspar, make up the other major igneous rock group of Earth’s crust.
Between these groups are rocks with an intermediate (andesitic) composition.
Ultramafic rocks, which lack light-colored minerals, lie at the far end of the
compositional spectrum from granitic rocks.
Factors Affecting Crystal Size:
Three factors influence the textures of igneous rocks:
(1) the rate at which molten rock cools;
(2) the amount of silica present; and
(3) the amount of dissolved gases in the magma.
Among these, the rate of cooling tends to be the dominant factor.
A very large magma body located many kilometers beneath Earth’s surface will cool
over a period of perhaps tens to hundreds of thousands of years. Initially, relatively
few crystal nuclei form. Slow cooling permits ions to migrate freely until they
eventually join one of the existing crystalline structures. Consequently, slow cooling
promotes the growth of fewer but larger crystals.
On the other hand, when cooling occurs rapidly—for example, in a thin lava
flow—the ions quickly lose their mobility .
Silica Content as an Indicator of Composition:
An important aspect of the chemical composition of igneous rocks is silica
(SiO2) content. Typically, the silica content of crustal rocks ranges from a low of
about 40 percent in ultramafic rocks to a high of more than 70 percent in granitic
rocks (see Figure 3.3). The percentage of silica in igneous rocks actually varies in a
systematic manner that parallels the abundance of other elements. For example,
rocks that are relatively low in silica contain large amounts of iron, magnesium, and
calcium.
By contrast, rocks high in silica contain very little iron, magnesium, or calcium but
are enriched with sodium and potassium.
Consequently, the chemical makeup of an igneous rock can be inferred directly from
its silica content. Further, the amount of silica present in magma strongly influences
its behavior.
Granitic magma, which has a high silica content, is quite viscous (“thick”) and may
erupt at temperatures as low as 700 °C. On the other hand, basaltic magmas are low
in silica and are generally more fluid.
Basaltic magmas also erupt at higher temperatures than granitic magmas—usually
at temperatures between 1100 and 1250 °C and are completely solid when cooled to
1000 °C. In summary, igneous rocks can be divided into broad groups according to
the proportions of light and dark minerals they contain. Granitic (felsic) rocks, which
are composed almost entirely of the light colored minerals quartz and feldspar, are
at one end of the compositional spectrum (see Figure 3.3). Basaltic (mafic) rocks,
which contain abundant dark silicate minerals in addition to plagioclase feldspar,
make up the other major igneous rock group of Earth’s crust. Between these groups
are rocks with an intermediate (andesitic) composition. Ultramafic rocks, which lack
light-colored minerals, lie at the far end of the compositional spectrum from granitic
rocks.
Igneous Rocks and Intrusive Activity in Review
• Igneous rocks form when magma cools and solidifies. Extrusive, or volcanic,
igneous rocks result when lava cools at the surface. Magma that solidifies at
depth produces intrusive, or plutonic, igneous rocks.
• As magma cools, the ions that compose it arrange themselves into orderly
patterns—a process called crystallization. Slow cooling results in the
formation of relatively large crystals.
• Conversely, when cooling occurs rapidly, the outcome is a solid mass
consisting of tiny inter grown crystals. When molten material is quenched
instantly, a mass of unordered atoms, referred to as glass, forms.
• The mineral composition of an igneous rock is the consequence of the
chemical make-up of the parent magma and the environment of
crystallization. Igneous rocks are divided into broad compositional groups
based on the percentage of dark and light silicate minerals they contain. Felsic
rocks (e.g., granite and rhyolite) are composed mostly of the light-colored
silicate minerals potassium feldspar and quartz. Rocks of intermediate
composition, (e.g., andesite and diorite) are rich in plagioclase feldspar and
amphibole. Mafic rocks (e.g., basalt and gabbro) contain abundant olivine,
pyroxene, and calcium-rich plagioclase feldspar. They are high in iron,
magnesium, and calcium, low in silica, and dark gray to black in color.
• The texture of an igneous rock refers to the overall appearance of the rock
based on the size, shape, and arrangement of its mineral grains. The most
important factor influencing the texture
• of igneous rocks is the rate at which magma cools. Common igneous rock
textures include aphanitic (fine-grained), with grains too small to be
distinguished without the aid of a microscope; phaneritic (coarse-grained),
with intergrown crystals that are roughly equal in size and large enough to be
identified without the aid of a microscope; porphyritic, which has larger
crystals (phenocrysts) embedded in a matrix of smaller crystals (groundmass);
and glassy.
• The mineral make-up of an igneous rock is ultimately determined by the
chemical composition of the magma from which it crystallizes. N. L. Bowen
di covered that as magma cools in the laboratory, those minerals with higher
melting points crystallize before minerals with lower melting points. Bowen’s
reaction series illustrates the sequence of mineral formation within magma.
• During the crystallization of magma, if the earlier-formed minerals are more
dense than the liquid portion, they will settle to the bottom of the magma
chamber during a process called crystal settling. Owing to the fact that
crystal settling removes the earlier formed minerals, the remaining melt will
form a rock with a chemical composition that is different from the parent
magma. The process of developing more than one magma type from a
common magma is called magmatic differentiation.
• Once a magma body forms, its composition can change through the
incorporation of foreign material, a process termed assimilation, or by
magma mixing.
• Magma originates from essentially solid rock of the crust and mantle. In
addition to a rock’s composition, its temperature, depth (confining pressure),
and water content determine whether it exists as a solid or liquid. Thus,
magma can be generated by raising a rock’s temperature, as occurs when a
hot mantle plume “ponds” beneath crustal rocks. A decrease in pressure can
cause decompression melting. Further, the introduction of volatiles (water)
can lower a rock’s melting point sufficiently to generate magma. A process
called partial melting produces a melt made of the low-melting-temperature
minerals, which are higher in silica than the original rock. Thus, magmas
generated by partial melting are nearer to the felsic end of the compositional
spectrum than are the rocks from which they formed.
• Intrusive igneous bodies are classified according to their shape and by their
orientation with respect to the country or host rock, generally sedimentary or
metamorphic rock. The two general shapes are tabular (sheet-like) and
massive. Intrusive igneous bodies that cut across existing sedimentary beds
are said to be discordant; those that form parallel to existing sedimentary
beds are concordant.
• Dikes are tabular, discordant igneous bodies produced when magma is
injected into fractures that cut across rock layers. Nearly horizontal, tabular,
concordant bodies, called sills, form
• when magma is injected along the bedding surfaces of sedimentary rocks. In
many respects, sills closely resemble buried lava flows. Batholiths, the
largest intrusive igneous bodies, sometimes make up large linear mountains,
as exemplified by the Sierra Nevada. Laccoliths are similar to sills but form
from less fluid magma that collects as a lens-shaped mass that arches
overlying strata upward.
• Some of the most important accumulations of metals, such as gold, silver,
lead, and copper, are produced by igneous processes. The best-known and
most important ore deposits are generated from hydrothermal (hot-water)
solutions. Hydrothermal deposits are thought to originate from hot, metalrich fluids that are remnants of late-stage magmatic processes. These ionrich solutions move along fractures or bedding planes, cool, and precipitate
the metallic ions to produce vein deposits. In a disseminated deposit (e.g.,
much of the world’s copper deposits), the ores from hydrothermal solutions
are distributed as minute masses throughout the entire rock mass.